In modern industrial practice, a variety of highly exothermic reactions are known to be promoted by contacting of the reaction mixture in the gaseous or vapor phase with a heterogeneous catalyst. In some cases these exothermic reactions are carried out in catalyst-containing structures or vessels where external cooling must be supplied, in part, because of the inability to obtain sufficient heat transfer and the need to control the reaction within certain temperature constraints. In these cases, it is not considered practical to use a monolithic catalyst structure, where the unreacted portion of the reaction mixture supplies the cooling for the catalytic reaction, because existing catalyst structures do not provide an environment whereby the desired reaction can be optimized while removing the heat of reaction through heat exchange with unreacted reaction mixture under conditions where undesired reactions and catalyst overheating are avoided. Thus, the applicability of monolithic catalysts structures to many catalyzed exothermic reactions could clearly be enhanced if monolithic catalyst structures could be developed wherein the reaction zone environment and heat exchange between reacted and unreacted portions of the reaction mixture are improved.
There is also a clear need to improve the operability of monolithic catalyst structures in areas where they are currently used or proposed for use, such as the combustion or partial combustion of fuels or the catalytic treatment of exhaust emissions from internal combustion engines, to widen the range of operating, conditions at which the desired catalytic conversions can be achieved. For example, in the case of catalytic combustion when applied to reduce NO.sub.x emissions from a gas turbine by equipping the turbine with a catalytic combustor, a clear need exists for catalytic systems or structures which will adapt to a variety of operational situations. A gas turbine used as a power source to drive a load must be operated over a range of speeds and loads to adjust power output to the load requirements. This means that the combustor must operate over a range of air and fuel flows. If the combustor system uses a catalyst to combust the fuel and limit emissions, then this catalyst system must be able to operate over a wide range of air flows, fuel/air ratios (F/A) and pressures.
Specifically in the case of an electric power generation turbine where the rotational speed is constant because of the need to generate power at a constant frequency, the air flow over the load range of 0% to 100% will be approximately constant. However, the fuel flow will vary to match the load required so the F/A will vary. In addition, the pressure will increase somewhat as the power output is increased. This means that the catalytic combustor must operate over a wide range of F/A and a range of pressures but at relatively constant mass flow. Alternatively, a variable portion of the air flow can be bypassed around the combustor or bled from the gas turbine to decrease the air flow and maintain a more constant F/A. This will result in a narrower range of F/A over the catalyst but a wider range of mass flows.
Further, in the case of a variable speed turbine, or a multiple shaft turbine, the air flow and pressure can vary widely over the operating range. This results in a wide variation of total mass flow and pressure in the combustor. Similar to the situation described above for the electric power generation turbine, the air can be bypassed or bled to control the F/A range resulting in a combustor that must operate over a range of mass flows.
The situations described above result in the need for a catalyst design that can operate over a wide mass flow range, pressure range and F/A range.
One particular application that could benefit from catalytic combustion is a gas turbine applied to a vehicle to achieve very low emissions. Once started, this engine must operate from idle to full load and achieve low emissions over this entire range. Even if the gas turbine is used in a hybrid vehicle design combined with a storage component such as a battery, flywheel, etc., the engine must still operate at idle and full load and must transit between these two operating points. This requires operation at mass flows and pressures of both of these conditions.
The present invention employs a catalyst structure made up of a series of adjacently disposed catalyst-coated and catalyst-free channels for passage of a flowing reaction mixture, wherein the catalytic and non-catalytic channels share a common wall such that integral heat exchange can be used to dissipate the reaction heat generated on the catalyst and thereby control or limit the temperature of the catalyst. That is, the heat produced on the catalyst in any given catalyst-coated channel flows through the common wall to the opposite non-catalytic surface to be dissipated into the flowing reaction mixture in the adjacent catalyst-free channel. With the present invention, the configuration of the catalytic channels differs from the non-catalytic channels in one or more critical respects, including the tortuosity of the flow channel, such that, when applied to catalytic combustion, catalytic and homogeneous combustion is promoted within the catalytic channels and not promoted or substantially limited in the non-catalytic channels while heat exchange is otherwise optimized. These uniquely configured catalyst structures substantially widen the window of operating parameters for catalytic combustion and/or partial combustion processes.
The use of catalyst supports having integral heat exchange in catalyst-promoted combustion or partial combustion is known in the art. In particular, Japanese Kokai 59-136,140 (published Aug. 4, 1984) and Kokai 61-259,013 (published Nov. 17, 1986) disclose the use of integral heat exchange in either a square-sectioned ceramic monolithic catalyst support in which alternating longitudinal channels (or layers) have catalysts deposited therein, or a support structure made up of concentric cylinders in which alternating annular spaces in the support are coated with catalyst. In both cases, the design of the catalyst structure disclosed is such that the configuration of the catalyst-coated channels and catalyst-free channels is the same with the catalytic and non-catalytic flow channels in each case being essentially straight and of the same cross-sectional area throughout their lengths.
A disclosure very similar to the two Japanese Kokai is seen in U.S. Pat. No. 4,870,824 to Young et al. where integral heat exchange is employed is a honeycomb support structure in which the catalyst-coated and catalyst-free channels are of identical configuration, being essentially straight and of unvarying square cross-sectional area throughout their length.
More recently, a series of U.S. patents have issued to Dalla Betta et al., including U.S. Pat. Nos. 5,183,401; 5,232,357; 5,248,251; 5,250,489 and 5,259,754, which describe the use of integral heat exchange in a variety of combustion or partial combustion processes or systems, including those where partial combustion of the fuel occurs in a integral heat exchange structure followed by subsequent complete combustion after the catalyst. Of these U.S. patents, U.S. Pat. No. 5,250,489 seems most in point, being directed to a metallic catalyst support made up of a high temperature resistant metal formed into a multitude of longitudinal passageways for passage of a combustible gas, with integral heat exchange being employed between passageways at least partially coated with catalyst and catalyst-free passageways to remove heat from the catalytic surface in the catalyst-coated passageways. The catalytic support structures disclosed in U.S. Pat. No. 5,250,489, include structures (FIGS. 6A and 6B) of U.S. Pat. No. 5,250,489 wherein the combustible gas passageways or channels are formed by alternating broad or narrow corrugations of a corrugated metal foil such that the size of the alternating catalytic and non-catalytic channels are varied to allow 80% of the gas flow to pass through the catalytic channels and 20% through the non-catalytic channels in one case (FIG. 6A), or 20% of the gas flow to pass through the catalytic channels and 80% through the non-catalytic channels in the other case (FIG. 6B). Using different sized channels as a design criterion, this patent teaches that any level of combustible gas conversion to combustion products between 5% and 95% can be achieved while incorporating integral heat exchange. While this patent does disclose the use of different sized catalytic and non-catalytic channels to vary the level of conversion, it clearly does not contemplate the use of channels having different tortuosity in the catalytic versus non-catalytic channels to optimize the combustion reaction in catalytic channels while substantially limiting homogeneous combustion in the non-catalytic channels as a means of widening the range of process conditions under which the catalyst structure can effectively operate.
In cases where the integral heat exchange structure is used to carry out catalytic partial combustion of a fuel followed by complete combustion after the catalyst, the catalyst must burn a portion of the fuel and produce an outlet gas sufficiently hot to induce homogeneous combustion after the catalyst. In addition, it is desirable that the catalyst not become too hot since this would shorten the life of the catalyst and limit the advantages to be gained from this approach. As the operating condition of the catalyst is changed, it is noted with the integral heat exchange structures of the prior art, discussed above, that operating window of such catalysts are limited. That is, that the gas velocity or mass flow rate must be within a certain range to prevent catalyst overheating.
Therefore, it is clear that a need exists for improved catalytic structures employing integral heat exchange which will substantially widen the window or range of operating conditions under which such catalytic structures can be employed in highly exothermic processes like catalytic combustion or partial combustion. The present invention capitalizes on certain critical differences in the configuration of the catalytic and non-catalytic passageways or channels in an integral heat exchange structure to materially widen the operating window for such catalysts.